Numerical simulation of aqueous ammonia-based CO2 absorption in a sprayer tower: An integrated model combining gas-liquid hydrodynamics and chemistry

Abstract

CO2 capture in the spray tower using aqueous ammonia solution has been proved a potential approach for the post-combustion CO2 capture. To improve the performance of spray absorption, a new three-dimensional numerical model integrating the complex gas-liquid hydrodynamics and the chemistry of the NH3-CO2-H2O system has been proposed and developed in this paper. In the model, the gas-liquid hydrodynamics was simulated using the Euler-Lagrange method, while the chemistry of the NH3-CO2-H2O system, including both thermodynamic and kinetic characteristics, was modeled using user-defined modules implemented in the CFD package. Moreover, a laboratory scale spray absorption system was built, and run under various operating conditions were carried out in order to validate the model developed. Subsequently, the predicted results using different kinetic models were compared with the experimental results. It was found the model using Liu’s kinetics showed a better agreement with the experiments, especially when the liquid temperature and NH3 concentration of solution vary. Furthermore, the validated model was used to study the fundamentals of spray absorption. It has been found that the gas temperature profile as well as gaseous CO2 and NH3 concentration profile depends on the velocity field, indicating the hydrodynamics should be considered when optimizing the design of reactor. It has also been found that the residence time of droplets increases with the increase of gas flow rate, leading to an increase in average droplets temperature and CO2 loading, while the increase of ammonia concentration of solution could significantly improve the overall CO2 mass transfer coefficient. When the solution temperature increases from 293 K to 303 K, the overall CO2 mass transfer coefficient gradually increases, resulting in CO2 removal efficiency improvement. However, when the solution temperature increases from 303 K to 313 K, the promotion of reverse reaction leads to the reduction of CO2 removal efficiency. In addition, the increase of ammonia solution flow rate would mainly increase the interphase area and promote both CO2 absorption and NH3 escape. The model developed in this paper can be used to optimize the design and operation of such reactors in practice.